I enabled administrative functionality as described there and provided the credentials of the default "Administrator" user. However, after entering a workspace folder name, NWDS just reports: "(Could not create workspace folder) [Error DI.dtr.253003]"

-You are using NWDS 7.5-The User you are using to create the folder in NWDS is added to group NWDI.Administrators-The DTR Perspective has switched to administrative mode.

If none of the previous hints helped I would ask to provide some screenshots how you are trying to create the WS folder and I can suggest to check the NWDS .log file what you can reach via HELP--> Support information (Then select a path where a supportinformation.zip file will be generated.). In this .zip you can find the .log file where you can find further information from the errors which are documented by NWDS.

It might be also useful to share the error from the .log file with us.